Field of the Invention
This invention relates to input waveform follow-up AC power source systems and, more particularly, to AC power sources which receive commercial AC power and can control their output voltage or power continuously from the neighborhood of zero voltage to the neighborhood of the input voltage while holding sinusoidal voltage waveform. Prior Art
FIGS. 14A and 14B show examples of well-known pertaining means for obtaining desired AC voltage or the like from commercial AC power. In the FIG. 14A example, AC power input between terminals 1 and 2 is converted to a desired voltage using a slidac transformer ST. In the FIG. 14B example, a triac TR1 is used for phase angle control of the input power, or thyristors SCR1 and SCR2 are used to provide an AC source which is applied to light controllers or electric furnace control.
The slidac ST permits obtaining a continuously variable sinusoidal output and finds extensive applications to equipment for researches and experiments. On the demerit side, since it is principally an auto-transformer used for commercial frequencies, it is large in volume and heavy in weight. Beside, it has resort to the sliding of coils for voltage variation, resulting in the generation of an inter-coil step voltage. Therefore, it is impossible to obtain perfectly continuously variable voltage. A further demerit is its high price.
The power source using the triac TR1 is small in size, light in weight and inexpensive, so that it is extensively applied to illumination units and electric furnaces as well as household devices. However, lagging power factor always results from reduction of current, voltage or power for the phase angle control. This gives rise to the generation of the third and higher odd harmonic waves by about 30% of the fundamental AC component at phase angle of 90.degree. these harmonic components being liable to leak to the commercial power source line so as to adversely affect other electronic devices. Actually, this power source is a subject of harmonic wave restriction. In addition, the reduction of the power factor increases ineffective power, which is a problem from the standpoint of energy saving.